One conventional method of producing ground wood pulp for the manufacture of paper products involves pressing a batch of pulpwood (roundwood or wood chips) against a rotating grinding stone while simultaneously feeding shower water into the grinding chamber, specifically by spraying the water directly on the surface of the stone at a location spaced from the actual grinding location. By means of a dam or weir, the formed ground wood stock, which is an aqueous slurry of pulp, is kept in the grinding chamber at a level a little higher than the lower point of the stone in order to clean, lubricate and cool the stone. The ground wood stock flowing over the dam is discharged by its own weight for further treatment. A variant of the foregoing is the "pitless" method, in which the stone is not immersed, and provision is made for extra water showers.
Another known method utilizes a disc refiner, in which material being refined or reduced is worked between two closely spaced opposed discs which undergo relative rotation.
In a recent development, the wood is ground under superatmospheric pressure, thus permitting grinding temperatures higher than in the standard stone groundwood (SGW) In U.S. Pat. Nos. 3,808,090 and 3,948,449, a process is described for improving the groundwood pulp by grinding wood in a closed grinding chamber in a pressurized gaseous atmosphere. In the two patents just named, the wood is fed in and the superatmospheric pressure in the grinding chamber can be maintained only so long as the grinding of a wood batch continues. However, when a new wood batch must be fed into the magazine, the magazine must be opened and the pressure of the grinding surface falls to atmospheric. Thus, the grinder does not work in a continuously pressurized atmosphere.
In an attempt to overcome the problem just defined, additional developments have been made and patented by Oy Tampella Ab, as exemplified in Canadian Patent No. 1,097,118 issued Mar. 10, 1981, and U.S Pat. Nos. 4,270,703 and 4,274,600 issued June 2, 1981 and June 23, 1981, respectively. In the Oy Tampella process, a feed chamber upstream of the grinding chamber has two pressure seals, one to the atmosphere and one to the grinding chamber. Thus, the feed chamber acts as a double-lock seal, to allow the pressure in the grinding chamber always to be maintained above atmospheric. By the use of this method, the pressure in the grinding chamber may reach as high as several bar, and temperatures at the grinding stone surface may climb well above the standard pressure boiling point.
Because of the considerable size and complexity of the SGW process and the pressurized groundwood (PGW) process developed hitherto, it would be desirable to reduce the complexity and size of an installation for producing ground pulp that can be used in paper making. In both the PGW and SGW processes, very large pressure shoes must be hydraulically driven to urge the roundwood against the grinding stone, and above the general location of the pressure shoes must be provided a stack for the incoming wood to be ground.
A different approach to the grinding of wood pulp is one in which the grinding pressure between the wood and the grinding surface is brought about centrifugally, by providing an internal cylindrical grinding surface, and by "flinging" the wood outwardly against the stationary grinding surface through the use of centrifugal force. The centrifuging action not only would allow the appropriate pressure to arise between the wood and the grinding surface, but could also pressurize a quantity of water being swept around along with the wood, thus permitting higher temperatures than the maximum attainable in the standard SGW process.
Early Canadian Patent No. 2834, issued Oct. 24, 1873 discloses a primitive version of a centrifugal grinder for wood, which incorporates a stationary internal cylindrical grinding surface, and a rotor turning about a vertical axis, and flinging the feed wood centrifugally outward along radial pathways to contact the grinding surface. Water for cooling the grinding surface and for making up the pulp slurry is simply squirted into the housing by a single hose or pipe.
Because of the primitive construction utilized by Moore, his apparatus would not do for the high speed grinding requirements of the present day.
Accordingly, it is an aspect of this invention to provide an apparatus that is improved with respect to the Moore apparatus, and in particular which utilizes the rotating principle in order to promote uniform and pressurized water spray against the internal grinding surface.
"Accordingly, this invention provides a centrifugal grinder which has an internal grinding surface in the shape of a surface of revolution. A rotor is mounted for rotation coaxially with the grinding surface, and has a central cavity defining at least two pockets through which material in the central cavity can contact the grinding surface. The pockets are uniformly distributed around the rotor and are separated by intermediate regions. Material is delivered to the central cavity and the rotor is rotated. Means are incorporated in the rotor for applying water to the grinding surface, the latter means including a first water pathway into the rotor adjacent the axis thereof, and for each pocket a second water pathway in the rotor adjacent the grinding surface and trailing the respective pocket in the sense of rotation. Water passage means joins the first pathway to each second pathway, and nozzle means communicates with each second pathway for spraying water against the grinding surface. Each second pathway is further than the first pathway from the rotor axis, so that rotation of the rotor increases the water pressure in the nozzle means with respect to that in the first pathway, due to the centrifugal effect.
This invention further provides a method of grinding a wood material against an internal grinding surface in the shape of a surface of revolution. The method includes a first step of rotating the material around the internal grinding surface in a plurality of discrete and circumferentially separated pockets to generate centrifugal grinding force between the material and the surface. The grinding surface is sprayed with water from nozzle means adjacently behind each pocket in the sense of rotation, in order to remove wood fibers therefrom and create a slurry. The centrifugal effect is utilized to increase water pressure at the nozzle means."